U.S. Pat. No. 5,398,661 discloses a method of this kind which is based on an underpressure check. A method based on an overpressure check is disclosed in U.S. Pat. No. 5,349,935.
German patent publication 4,239,382 discloses that the diagnostic reliability for the check of the through-flow control valve is affected under specific operating conditions. The through-flow control valve controls the flow of fuel vapor from the tank-venting system to the intake manifold of the internal combustion engine. To ameliorate this condition, it is suggested in this patent publication to detect the absolute values and/or change of operating characteristic values which are relevant for the diagnosis and to compare the same to pregiven threshold values and to interrupt the diagnosis as may be necessary.
It has been shown that erroneous fault announcements (that is, those not based on leaks) can occur even in leakage diagnostic methods.